Prior studies demonstrated that practice of a skill or forceful voluntary muscle contractions on one side of the body produces functionally and clinically meaningful increases in motor output of the same muscle on the opposite side of the body. Such adaptations occur in the absence of muscle hypertrophy and thus implicate a role for the ipsilateral motor cortex and the spinal cord in force transfer. In a series of experiments we will characterize the inhibitory and excitatory mechanisms of interhemispheric practice-induced transfer of muscle force. We hypothesize that the effects of unilateral practice on interhemispheric inhibition and on the excitability of the contralateral corticospinal projections are linked and these effects are graded according to the nature of muscle activation. We will use single and multiple pulse and repetitive transcranial magnetic brain stimulation to explore the interhemispheric mechanisms of force transfer and peripheral nerve stimulation to determine if spinal mechanisms also contribute to force transfer induced by voluntary, imagined, and electrically stimulated muscle contractions in healthy young humans. The results of these studies will substantially increase our understanding of CNS control of voluntary movement.